(a) Technical Field
The present invention relates to a humidification system for a fuel cell. More particularly, the present invention relates to a humidification system for a fuel cell which can adjust humidification of dry air to be supplied to a fuel cell stack.
(b) Background Art
A fuel cell system is an electricity generation system that converts chemical energy of fuel directly into electric energy.
The fuel cell system generally comprises a fuel cell stack for generating electricity, a fuel supply system for supplying fuel (hydrogen) to the fuel cell stack, an air supply system for supplying oxygen in air, which is an oxidizing agent required for en electrochemical reaction, to the fuel cell stack, and a heat and water management system for removing reaction heat of the fuel cell stack to the outside of the fuel cell system and controlling the operation temperature of the fuel cell stack.
The fuel cell system having the above configuration generates electricity by the electrochemical reaction of hydrogen as fuel and oxygen in the air and exhausts heat and water as reaction by-products.
Meanwhile, the fuel cell system requires humid air to facilitate the chemical reaction. The humidification is carried out by directly supplying water to the fuel cell stack by using a hollow fiber membrane or a membrane in the form of a film, or by exchanging moisture of supersaturated humid air discharged from the fuel cell stack with dry air from the outside.
FIG. 1 is a diagram showing the state where humid air is supplied by a conventional humidification system for a fuel cell. As shown in the figure, the dry air from the outside is forcibly blown by a blower 1 to pass through a membrane humidifier 2 and supersaturated humid air discharged from an outlet of a fuel cell stack 3 is passed through the membrane humidifier 2 (then discharged to the outside air) such that the humidification is achieved by moisture exchange between the supersaturated humid air and the dry air. The thus humidified air is supplied to the fuel cell stack 3.
The membrane humidifier 2 is a gas-to-gas membrane humidifier employing hollow fiber membranes. In the event that such a membrane humidifier 2 is used, since it is possible to highly integrate hollow fiber membranes having a large contact surface area, a sufficient humidification of the fuel cell stack 3 is achieved even with a small capacity. Moreover, since the moisture and heat contained in the unreacted gas discharged at a high temperature from the fuel cell stack 3 are collected and reused, it is possible to save the moisture and energy required for the humidification of the fuel cell stack 3.
One of the factors directly influencing the operation performance of the fuel cell system is to water content. Particularly, more than a predetermined amount of moisture must be supplied to an electrolyte membrane and an ionomer in a catalyst layer of a membrane electrode assembly (MEA) to obtain the maximum performance of the ion conductivity of the electrolyte membrane and the ionomer.
Here, the function of the membrane humidifier 2 is to supply the moisture and heat contained in the unreacted gas discharged at a high temperature from the fuel cell stack 3 to dry gas at a room temperature supplied to the fuel cell stack 3 through the membrane surface, thus achieving the humidification and temperature maintenance of the fuel cell stack 3.
Conventional membrane humidifiers mainly comprise a bundle of hollow fiber membranes through which dry air passes and a housing through which the supersaturated humid air discharged from the fuel cell stack 3 passes. In this case, the housing has a substantially cylindrical shape including an inlet and an outlet through which the dry air and the supersaturated humid air pass.
However, the conventional humidification systems have the following drawbacks.
First, humidification more than a certain level can increase the amount of water generated from a cathode in a high current and high output region while it is suitable for a low current and low output region.
The increased amount of water generated from the cathode in the high current and high output region can increase the material transfer resistance at the cathode, causing a flooding problem. This, in turn, can block the supply of air at the cathode and cause a problem of starvation of air. As a result, deterioration of the fuel cell catalyst can be accelerated and durability of the fuel cell can be reduced remarkably.
Accordingly, in the high current and high output region, it is necessary to reduce the humidification amount to a certain level. Nonetheless, the conventional membrane humidifiers show little difference in the humidification amount between the low current region and the high current region; the high current region has a humidification performance of more than RH 80% which is a level similar to the low current region. Moreover, the conventional membrane humidifiers have drawbacks in that in the high current region, with an increase in the amount of air flow, a pressure drop is increased and the load of the blower is raised.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.